The present invention relates to a uniaxially high-oriented polyethylene naphthalate film for liquid crystal panel substrates and, in particular, to a film suitable for a cell of a flexible liquid crystal panel. More particularly, the present invention relates to a polyethylene naphthalate film useful as a cell of a flexible liquid crystal panel, which is stretched uniaxially with high orientation, and has an in-plane birefringence of more than 0.110 after the film is stretched and subjected to heatset treatment.
Uniaxially stretched polyethylene terephthalate films having good mechanical properties, electrical properties, heat resistance and chemical resistance, have conventionally been used as cells of liquid crystal panels. A uniaxially stretched polyethylene terephthalate film, however, has been found to have various defects as the film has increasingly been used for wide purposes, and an improvement has been demanded.
For example, when a uniaxially stretched polyethylene terephthalate film is used in place of a glass substrate for a liquid crystal panel, it is difficult to completely orient in one direction, thereby having a problem in terms of extinction, and since the heat resistance thereof is low, it is impossible to raise the temperature when manufacturing the film. Such a uniaxially stretched polyethylene terephthalate film also involves a critical defect; when the film is exposed to a high temperature, for example, when a transparent electrode is formed by vacuum evaporation, an oligomer educes, thereby deteriorating the transparency of the film, disconnecting the lead wire of the transparent electrode, or increasing the resistance of the lead wire thereof. If the orientation of one direction is increased to improve the loss of the extinction, the film is likely to be torn in the orientation direction, thereby enhancing notched effect and extremely reducing the manufacturing yield.
In place of such polyethylene terephthalate films, polyethylene naphthalate films have been proposed. For example, Japanese Patent Laying-Open (KOKAI) No. 50-45877 (1975) discloses a biaxially oriented polyethylene 2,6-naphthalate-film having a Young's modulus in the longitudinal direction of not less than 51,000 kg/cm.sup.2, and a Young's modulus in the transverse direction of not less than 68,000 kg/cm.sup.2, and which is useful as an electrical insulating material, a base material for magnetic recording tape. Japanese Patent Publication No. 56-19012 (1981) discloses a magnetic recording tape comprising a magnetic layer formed on the surface of a biaxially oriented polyethylene-2,6-naphthalate film which has a Young's modulus in the longitudinal direction of not less than 51,000 kg/cm.sup.2 and a Young's modulus in the transverse direction of not less than 68,000 kg/cm.sup.2. Japanese Patent Publication No. 48-29541 (1973) discloses a method of manufacturing a magnetic recording tape having a large tensile strength in the longitudinal direction and a good dimensional stability comprising a biaxially stretched polyethylene naphthalate film and a mixture of magnetic particles and a binder coated on the surface thereof.
However, no suggestion has ever been made as to the use of a uniaxially stretched polyethylene naphthalate film as a liquid crystal panel substrate.
It is an object of the present invention to provide a film for liquid crystal panel substrates which is superior in weather resistance, tear strength and heat resistance to a stretched polyethylene terephthalate film and, which produces no education (bleeding) of a low-molecular weight polymer such as an oligomer.
As a result of researches on how to achieve this aim, the present inventor has found that it is very difficult to obtain a film having the above-described properties by using a polyethylene terephthalate film but it is enabled by using a polyethylene naphthalate film which is uniaxially highly oriented more than a predetermined value, and the present invention has been achieved on the basis of this finding.